The present invention relates to a frame-supported pellicle for photolithography or, more particularly, to a pellicle consisting of a rigid frame and a pellicle membrane supported by the frame in a slack-free fashion as being adhesively bonded thereto and used for dust-proof protection of a photomask used in the photolithographic patterning work of various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal display units and the like as well as to a method for the preparation thereof.
As is well known, the so-called photolithographic patterning work is widely undertaken in the manufacture of various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal display units and the like, in which the photoresist layer on the substrate such as semiconductor silicon wafers and base plates of liquid-crystal display units is exposed pattern-wise to actinic rays such as ultraviolet light through a so-called photomask which is a transparency bearing the fine pattern to be formed in the photoresist layer. The pattern on the photomask is so fine that any dust particles deposited on the photomask greatly affect the quality of pattern reproduction resulting in a decrease in the yield of acceptable products.
It is a usual practice accordingly that the photolithographic patterning work is conducted in a clean room freed from dust particles floating in the air as completely as possible but perfect cleanness of photomasks can hardly be obtained even in a clean room of the highest degree of cleanness. Therefore, a conventional means to ensure cleanness of a photomask is to protect the photomask by mounting a frame-supported pellicle thereon, which consists of a rigid frame to be mounted on and fixed to the photomask by means of a pressure-sensitive adhesive and a highly transparent pellicle membrane of a plastic resin adhesively bonded to the frame in a slack-free fashion so that dust particles are never deposited on the surface of the photomask but are deposited on the pellicle membrane. The dust particles deposited on the pellicle membrane have little adverse influences on the quality of pattern reproduction since the light for exposure is focused on the photomask and not on the pellicle membrane at a distance from the photomask.
Needless to say, a primary requirement for the pellicle membranes is that transmission of light for the pattern-wise exposure therethrough is as high as possible. In this regard, several polymeric materials such as nitrocellulose, cellulose acetate and the like are widely used as a material of pellicle membranes. In connection with the method for adhesive bonding of a pellicle membrane to a frame, Japanese Patent Kokai No. 58-219023 proposes that a pellicle membrane made from these polymeric materials is attached and bonded to the surface of a pellicle frame, which is made from aluminum, stainless steel, polyethylene and the like, wet with an organic solvent capable of dissolving the polymer of the membrane. Alternatively, the pellicle membrane can be adhesively bonded to the surface of a pellicle frame by using an acrylic or epoxy resin-based adhesive (see, for example, U.S. Pat. No. 4,861,402 and Japanese Patent Publication No. 63-27707). Besides the above mentioned cellulose derivatives as a material of the pellicle membrane, a proposal has been recently made for the use of an amorphous fluorocarbon polymer. The lower surface of the pellicle frame, i.e. the surface opposite to the surface to which the pellicle membrane is adhesively bonded, is usually coated with a pressure-sensitive adhesive based on a polybutene resin, polyvinyl acetate resin, acrylic resin and the like in order to facilitate mounting of the pellicle on a photomask. Commercial products of frame-supported pellicles are usually provided with a release paper sheet to temporarily protect the adhesive layer on the lower surface of the frame from inadvertent sticking and to ensure easy handling thereof.
The adhesive, by which the pellicle frame and the pellicle membrane are adhesively bonded together, also must satisfy various requirements. In addition to a high adhesive bonding strength obtained therewith to ensure stability of the supported state of the membrane on the frame, the adhesive is required to be highly resistant against irradiation with ultraviolet light because the adhesive layer between the pellicle membrane and the frame is directly exposed to the ultraviolet light during the photolithographic patterning process since otherwise the adhesive causes degradation after a relatively short time of service under ultraviolet light to become brittle so that the thus embrittled adhesive per se produces dust particles if not to mention the decrease in the adhesive bonding strength eventually leading to separation of the membrane from the frame. Conventional acrylic and epoxy resin-based adhesives are not quite satisfactory in this regard. When the pellicle membrane is formed from an amorphous fluorocarbon polymer, no satisfactory adhesive bonding strength can be obtained between the frame and the membrane because fluorocarbon polymers are generally little susceptible to adhesive bonding with a conventional adhesive based on an acrylic or epoxy resin.